EP0329603B1 - Grain boundary junction devices using high-tc superconductors - Google Patents
Grain boundary junction devices using high-tc superconductors Download PDFInfo
- Publication number
- EP0329603B1 EP0329603B1 EP89810047A EP89810047A EP0329603B1 EP 0329603 B1 EP0329603 B1 EP 0329603B1 EP 89810047 A EP89810047 A EP 89810047A EP 89810047 A EP89810047 A EP 89810047A EP 0329603 B1 EP0329603 B1 EP 0329603B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- superconducting
- grain boundary
- devices
- regions
- layer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 239000002887 superconductor Substances 0.000 title claims description 34
- 239000000463 material Substances 0.000 claims description 74
- 239000000758 substrate Substances 0.000 claims description 43
- 238000000034 method Methods 0.000 claims description 38
- 241000238366 Cephalopoda Species 0.000 claims description 20
- QPLDLSVMHZLSFG-UHFFFAOYSA-N Copper oxide Chemical compound [Cu]=O QPLDLSVMHZLSFG-UHFFFAOYSA-N 0.000 claims description 11
- 238000000059 patterning Methods 0.000 claims description 11
- 239000005751 Copper oxide Substances 0.000 claims description 9
- 229910000431 copper oxide Inorganic materials 0.000 claims description 9
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 6
- 150000002910 rare earth metals Chemical class 0.000 claims description 5
- 230000008878 coupling Effects 0.000 claims description 4
- 238000010168 coupling process Methods 0.000 claims description 4
- 238000005859 coupling reaction Methods 0.000 claims description 4
- 238000005036 potential barrier Methods 0.000 claims description 3
- 239000010410 layer Substances 0.000 description 41
- 239000010408 film Substances 0.000 description 33
- 230000004888 barrier function Effects 0.000 description 19
- 239000013078 crystal Substances 0.000 description 9
- 230000007704 transition Effects 0.000 description 9
- 229910002370 SrTiO3 Inorganic materials 0.000 description 8
- 229960004643 cupric oxide Drugs 0.000 description 7
- 230000008569 process Effects 0.000 description 7
- 238000000137 annealing Methods 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 6
- 238000012545 processing Methods 0.000 description 6
- 230000005641 tunneling Effects 0.000 description 6
- 238000000151 deposition Methods 0.000 description 5
- 230000008021 deposition Effects 0.000 description 5
- 230000006870 function Effects 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 5
- 238000005245 sintering Methods 0.000 description 5
- 229910003098 YBa2Cu3O7−x Inorganic materials 0.000 description 4
- 238000003491 array Methods 0.000 description 4
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 4
- AYJRCSIUFZENHW-UHFFFAOYSA-L barium carbonate Chemical compound [Ba+2].[O-]C([O-])=O AYJRCSIUFZENHW-UHFFFAOYSA-L 0.000 description 4
- 238000000608 laser ablation Methods 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 239000001301 oxygen Substances 0.000 description 4
- 229910052760 oxygen Inorganic materials 0.000 description 4
- 239000000843 powder Substances 0.000 description 4
- 239000010409 thin film Substances 0.000 description 4
- 230000008901 benefit Effects 0.000 description 3
- 230000001427 coherent effect Effects 0.000 description 3
- 230000000875 corresponding effect Effects 0.000 description 3
- 238000000407 epitaxy Methods 0.000 description 3
- 230000001747 exhibiting effect Effects 0.000 description 3
- 238000006303 photolysis reaction Methods 0.000 description 3
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 2
- 229910052788 barium Inorganic materials 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000010884 ion-beam technique Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 239000008188 pellet Substances 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 238000005507 spraying Methods 0.000 description 2
- 229910015901 Bi-Sr-Ca-Cu-O Inorganic materials 0.000 description 1
- 229910002480 Cu-O Inorganic materials 0.000 description 1
- 229910009203 Y-Ba-Cu-O Inorganic materials 0.000 description 1
- 238000002679 ablation Methods 0.000 description 1
- 230000004913 activation Effects 0.000 description 1
- 229910052785 arsenic Inorganic materials 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 230000005493 condensed matter Effects 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000008020 evaporation Effects 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 230000004907 flux Effects 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 230000008570 general process Effects 0.000 description 1
- 238000007731 hot pressing Methods 0.000 description 1
- BDAGIHXWWSANSR-NJFSPNSNSA-N hydroxyformaldehyde Chemical compound O[14CH]=O BDAGIHXWWSANSR-NJFSPNSNSA-N 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 239000012212 insulator Substances 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 229910052743 krypton Inorganic materials 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 238000001459 lithography Methods 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- -1 oxygen Chemical class 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 238000012827 research and development Methods 0.000 description 1
- 238000012552 review Methods 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 239000002356 single layer Substances 0.000 description 1
- 238000003746 solid phase reaction Methods 0.000 description 1
- 238000010671 solid-state reaction Methods 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- 229910000018 strontium carbonate Inorganic materials 0.000 description 1
- VEALVRVVWBQVSL-UHFFFAOYSA-N strontium titanate Chemical compound [Sr+2].[O-][Ti]([O-])=O VEALVRVVWBQVSL-UHFFFAOYSA-N 0.000 description 1
- 239000013589 supplement Substances 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 239000004408 titanium dioxide Substances 0.000 description 1
- 238000007740 vapor deposition Methods 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N60/00—Superconducting devices
- H10N60/10—Junction-based devices
- H10N60/12—Josephson-effect devices
- H10N60/124—Josephson-effect devices comprising high-Tc ceramic materials
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N60/00—Superconducting devices
- H10N60/01—Manufacture or treatment
- H10N60/0912—Manufacture or treatment of Josephson-effect devices
- H10N60/0941—Manufacture or treatment of Josephson-effect devices comprising high-Tc ceramic materials
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S505/00—Superconductor technology: apparatus, material, process
- Y10S505/70—High TC, above 30 k, superconducting device, article, or structured stock
- Y10S505/701—Coated or thin film device, i.e. active or passive
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S505/00—Superconductor technology: apparatus, material, process
- Y10S505/70—High TC, above 30 k, superconducting device, article, or structured stock
- Y10S505/701—Coated or thin film device, i.e. active or passive
- Y10S505/702—Josephson junction present
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S505/00—Superconductor technology: apparatus, material, process
- Y10S505/725—Process of making or treating high tc, above 30 k, superconducting shaped material, article, or device
- Y10S505/728—Etching
Definitions
- This invention relates to devices employing high-T c superconducting materials, and more particularly to simple, practical devices employing these materials, and to methods for making these devices.
- the devices are planar structures employing selected grain boundaries in the high-T c superconducting materials as weak link or junction barriers. Such structures are reproducibly made with good operating margins.
- Materials including the so called "1-2-3" phase in the Y-Ba-Cu-O system have been found to exhibit a superconducting transition temperature in excess of 77K.
- Laibowitz and co-workers were the first to achieve and describe a method for making thin films of these materials. These thin film structures and methods for making them are described in Phys. Rev. B, 35, 8821 (1987). In this technique, a vapor transport process is used in which the components of the superconducting film are vaporized and deposited on a substrate in an oxygen atmosphere, after which the deposited film is further annealed.
- references generally describing the deposition of films or layers of high-T c superconducting materials include EP-A-0 288 711 and EP-A-88810600.2.
- the first of these references describes a plasma spray coating process while the second describes a method for coating a substrate, as by spraying from solution, and then patterning the coated film to eventually produce a patterned layer of high-T c superconducting material.
- the copper oxide superconducting materials exhibiting transition temperatures in excess of about 30K are generally known as "high-T c superconductors", and will be referred to in that manner throughout the specification. This designation is meant to include both the materials having rare earth or rare earth-like elements in their crystalline structure, as well as the more recently reported materials which do not contain rare earth or rare earth-like elements. Generally, all these materials are copper-oxide-based superconductors having Cu-O planes that appear to be primarily responsible for carrying the supercurrents, where the copper oxide planes are separate or in groups separated by the other elements in the compositions.
- high-T c superconducting copper oxides such as YBa2Cu3O 7-x
- YBa2Cu3O 7-x can be reproducibly prepared as thin films
- a well defined, all high-T c single junction exhibiting Josephson tunneling currents has not been successfully fabricated.
- two superconducting layers comprised of high-T c superconductors are separated by a thin (1 ... 5 nm) layer which operates as a tunnel barrier.
- An oxide material can be used for the barrier layer.
- the high Tc copper oxide superconductors whether fabricated as films or bulk samples, require annealing in an oxygen atmosphere at high temperatures, typically about 900°C.
- the geometrical constriction must have a dimension of the order of the coherence length in order to exhibit weak-link characteristics. Such narrow constrictions cannot be reliably produced.
- planar junctions it is also very difficult to reliably deposit tunnel barrier layers having thicknesses of the order of the coherence length (about 1 nm) of high-T c superconductors.
- junction devices or weak link devices are fabricated using a grain boundary between two high-T c superconducting grains. These grain boundaries are very narrow (about the order of the unit cell in the materials, i.e., about 1 nm, and their electrical properties (such as resistance) can be readily varied to provide different device properties.
- a planar structure is provided utilizing an epitaxial film of high-T c superconducting material deposited on a substrate having defined and predetermined grain boundaries therein. In this manner, the grain boundaries in the substrate are reproducibly formed in the epitaxial superconductor film. Stated another way, epitaxy maps the polycrystalline structure of the substrate into the high-T c superconductor film.
- the devices described in these references use the grain boundaries that randomly occur when a superconductor film is deposited on a substrate.
- These superconductors are generally designated BPB films because they are comprised of Ba, Pb, and Bi oxide combinations having a perovskite-type structure.
- BPB films because they are comprised of Ba, Pb, and Bi oxide combinations having a perovskite-type structure.
- These references do not teach a way to controllably make grain boundary junction devices whose characteristics can be well controlled and which can be reproducibly formed with uniform properties.
- these references describe devices in which a random formation of randomly oriented grains occurs in materials having low transition temperatures of about 13K.
- the devices of the present invention are made in an epitaxial layer of high-T c superconducting material.
- epitaxy is thought of with respect to single crystal material rather than polycrystalline materials of the type used for the substrate and the superconducting film in the devices of this invention.
- This invention relates to improved devices utilizing high-T c superconducting materials and uniform, reproducibly created grain boundaries in such films for the fabrication of the devices. It is recognized that grain boundaries have been utilized in the prior art as tunnel barriers in the work relating to BPB oxides, and that the possible presence of grain boundaries leading to Josephson tunneling currents was mentioned in EP-A-0 286 891 However, the present invention seeks to provide devices and methods for producing these devices which are controllable and reproducible to define grain boundary devices in high-T c Josephson materials having small coherence lengths. Further, the location, orientation, and number of these new grain boundary devices can be predetermined, and the properties of each of the devices can be adjusted during fabrication.
- a substrate is prepared having at least one grain boundary therein, which grain boundary is to be reproduced in an overlying layer of high-T c superconductor.
- the layer of high high-T c superconductor is then epitaxially deposited on the substrate in order to reproduce in the superconducting layer the grain boundary present in the substrate. This defines the location and orientation of the grain boundary in a controlled manner.
- the superconducting film is patterned to leave at least one superconducting region on each side of the grain boundary, these superconducting regions being used as electrodes for current flow across the grain boundary.
- High energy beams or excimer laser ablation can be used to define the superconducting regions that are to function as the electrodes for these superconducting devices.
- the superconducting regions are electrically contacted and appropriately biased to have current flow across the grain boundary which functions as a potential barrier.
- a plurality of devices of this type can be arranged in any manner to produce an array of such devices, a SQUID, etc.
- superconducting devices comprised of a single layer of high-T c superconducting material can be made in a planar geometry utilizing grain boundaries for tunnel barriers or weak link connections between superconducting grains.
- this can be done reproducibly and controllably since the grain boundaries can be produced in the superconducting layers in a manner in which the orientation and location of the grain boundary are predetermined.
- the general process steps include the preparation of the substrate having at least one grain boundary defined therein with respect to the orientation and location of the grain boundary.
- a high-T c superconducting layer is epitaxially deposited on the substrate (or on a thin interface layer epitaxially grown on the substrate) in order to produce in the superconducting layer a grain boundary corresponding in location and orientation to the grain boundary in the underlying substrate.
- the superconducting layer is patterned to leave regions of superconducting material on either side of the grain boundary in order to produce a device having superconducting regions (electrodes) separated by the grain boundary. Electrical contacts can then be made to the superconducting regions for connection to appropriate biasing sources.
- the properties of the grain boundary can be adjusted and multiple devices can be fabricated along a single grain boundary or along several grain boundaries.
- FIGS. 1-4 the general technique for producing a planar grain boundary device is illustrated.
- this technique there is no subsequent processing step which would interfere with the material properties of any of the component parts of the device, and the structure that is obtained is planar.
- the grain boundary functions as a Josephson tunneling barrier or weak link connection, and is typically about 1 nm in width in these high-T c superconducting materials. More generally, the grain boundary width is of the order of the unit cell of these high-T c superconductors.
- substrate 10 includes two single crystal grains A and B separated by a grain boundary GB. This grain boundary is approximately 1 nm in width and is schematically illustrated by the stipled region between the crystalline grains A and B.
- a layer 12 high-T c superconducting material has been epitaxially deposited on the substrate 10 using, for example, known techniques. These techniques include vapor deposition as by evaporation or sputtering from multisources as described in the aforementioned articles in the names of R. B. Laibowitz, P. Chaudhari and others. Because the superconducting layer 12 is epitaxially formed on the substrate 10, it will have crystalline regions A and B coextensive with those in the substrate 10, and a grain boundary GB having the same orientation and location as the grain boundary in the underlying substrate 10.
- the superconducting layer 12 is patterned, for example by using photons or high energy particles, represented by the arrows 14.
- This patterning can be done in a variety of ways, and is used to define regions of the superconducting layer 12 which are to be left in their superconducting state while the irradiated portions are either physically removed or converted to a nonsuperconducting (i .e., normal) state.
- a nonsuperconducting i .e., normal
- High energy beams providing energies in the range of about 250 keV - 2 or 3 MeV will typically provide enough damage to alter the properties of high-T c copper oxide superconductors.
- patterning can be accomplished by energy beam irradiation where the superconducting material is not totally converted to a nonsuperconducting state, but rather has its critical transition temperature T c lowered appreciably with respect to the nonirradiated regions of the superconducting layer.
- Another technique for patterning high-T c superconducting layers is the use of excimer ablative photodecomposition in which ultraviolet radiation impinges on the superconducting layer to ablate (i.e., blow away) the irradiated regions.
- Ablative photodecomposition will occur if the energy fluence of the UV radiation is sufficiently high that the threshold for ablative photodecomposition is exceeded. In this process, the ablation occurs with substantially no thermal effect to the surrounding nonirradiated regions. This is a particularly good technique, as the surrounding regions will have the same superconducting properties after patterning has occurred.
- FIG. 4 illustrates the structure that remains after patterning.
- a thin strip of the superconductor 12 is left on the substrate 10, the superconductor 12 including grains A and B separated by the grain boundary GB in the superconducting material.
- the superconductor 12 is a planar structure wherein grains A and B can be used for electrical contacts, the current flow being substantially normal to the plane of the grain boundary.
- the general steps of this process include the provision of a substrate having at least one grain boundary therein whose location and crystal orientation are predetermined, the epitaxial deposition of a layer of high-T c superconducting film on the substrate to establish in the superconducting film a grain boundary corresponding to that in the substrate, patterning of the superconductor to leave superconducting regions separated by a portion of the grain boundary, and contacting the superconducting regions with the appropriate electrical sources.
- FIG. 5 One example of the final structure is shown in FIG. 5.
- electrical contacts 16 are made to superconducting grains A and B and a bias source, represented by battery 18 is attached thereto for providing a current flow across the grain boundary GB in the superconducting layer.
- the substrate materials are selected from those materials on which an epitaxial layer of high-T c superconducting film can be deposited.
- suitable substrates include SrTiO3 substrates for epitaxial deposition thereon of high-T c superconducting materials such as YBa2Cu3O 7-x .
- Other suitable substrates will be apparent to those of skill in the art, the substrates being generally chosen to have sufficient lattice match with the desired high-T c superconducting material that the superconducting material can be epitaxially deposited thereon.
- a grain boundary with a controlled misorientation can be obtained by forming a bicrystal from two oriented single crystals.
- the bicrystal is grown by sintering two single crystal pieces in a powder compact. During sintering, the single crystal pieces grow at the expense of the smaller surrounding grains until the single crystals impinge on each other to form a single grain boundary.
- This technique has been used to form the bicrystal of SrTiO3 at a sintering temperature of 1600°C.
- a bicrystal can be formed by hot pressing two single crystal pieces together. While both techniques can be used to form a long, well-defined grain boundary, the advantage of the second method is the ability to form a straight grain boundary that is free of pores.
- the grain boundary forms a tunneling barrier or weak link between the superconducting grains to which electrical contact is made.
- the critical current density and tunneling characteristics of the grain boundary can also be modified with a low temperature (less than 400°C) annealing step in a controlled gas atmosphere.
- a low temperature (less than 400°C) annealing step in a controlled gas atmosphere.
- inert and reducing gasses such as He and Ar, as well as reactive gasses such as CO2 are effective for this purpose.
- An inert gas annealing step acts to reduce the oxygen content of the superconductor film.
- a CO2 anneal will promote the formation of BaCO3 in a film of YBa2Cu3O 7-x , where BaCO3 is an insulator.
- These large grain SrTiO3 substrates were prepared by sintering cold-pressed pellets of fine-grained powder (average grain size approximately 2.5 ⁇ m) in air at temperatures in the range 1600-1650°C, for at least 48 hours. These sintering conditions cause exaggerated grain growth to occur, which leads to the formation of very large grains in the dense pellets ( ⁇ / ⁇ th ⁇ 99%)
- the strontium titanate powder was prepared by reacting high purity powders of strontium carbonate and titanium dioxide at 1450°C until a single phase material is obtained.
- high-T c YBa2Cu3 films were epitaxially deposited onto the large grain polycrystalline SrTiO3 substrates.
- the details of superconductor film deposition and post deposition annealing are those which have been described previously by R.B. Laibowitz, P. Chaudhari, et al.
- the superconducting films were epitaxially aligned with the grain orientation of the substrate, resulting in a large-grained superconducting film. This is illustrated in FIG. 6 where the superconducting film 20 is epitaxially aligned with the substrate 22. Large grains A and B are produced in the superconducting film 20, where the grains are separated by the grain boundary GB.
- FIGS. 8 and 9A - 9C The electrical characteristics of the three lines I, II, and III are shown in FIGS. 8 and 9A - 9C.
- the critical current Ic is plotted against temperature for current flow in each of the three lines I, II and III. From this FIG. it is apparent that line II, containing a grain boundary, always has a lower critical current than that lines I and III.
- FIGS. 9A - 9C more dramatically illustrate the essential features of a Josephson weak link junction in line II, in contrast with lines I and III.
- These FIGS. plot critical current Ic versus applied magnetic field H for each of the three lines I, II and III.
- the plots in FIGS. 9A and 9C are similar, while the plot in FIG. 9B clearly illustrates the presence of the grain boundary junction in line II.
- the junction resistance here is of the order of a few ohms and its capacitance is estimated to be a fraction of 1 picofarad.
- the Stewart McCumber parameter is of the order of 1 for these samples. Therefore, the hysteresis in the I-V curves for these samples is quite small, in sharp contrast to the conventional overlap junctions with higher capacitance made by separating two superconducting layers with an insulating layer of, for example, an oxide material.
- the number of grain boundaries, their orientations and their locations in the superconducting layer are known in advance of patterning for device formation. This is a major distinction over the randomness with respect to location and orientation of prior art grain boundary devices using other types of superconductors. Another major distinction is with respect to the superconducting grain size that is used in the present invention in contrast with that of prior art grain boundary devices. In the present invention, the grain sizes are typically hundreds of micrometers, in contrast with an average of about 10 microns grain size for prior art devices. Because the grains in the present invention are very large, it is easy to isolate grain boundaries for patterning and formation of devices. This cannot be achieved in the prior art where the grain boundaries are extremely short and randomly oriented.
- Another advantage of the present devices in contrast with prior art grain boundary devices relates to the uniformity of properties along the grain boundaries.
- long grain boundaries can be produced so that the same grain boundary can be used when patterning several devices, or an array of devices. Since the use of the same grain boundary as a barrier in more than one device helps to ensure the uniformity of individual device properties, the quality of circuits and arrays produced from devices made in accordance with the present invention can be significantly greater than those using prior art structures.
- a superconducting loop is formed including at least 2 weak links or tunnel barriers.
- the present invention it is possible to construct the superconducting loop in such a way that multiple tunnel barriers are formed using the same grain boundary in order to make all the weak link devices in the superconducting loop have essentially identical properties. This cannot be achieved in prior art structures utilizing several grain boundaries.
- FIGS. 10A and 10B illustrate a SQUID device that was prepared in accordance with the present invention
- FIG. 11 is a plot of critical current Ic versus applied magnetic field H, for different values of bias current through the SQUID. This plot was made at 4,6K, although the same general characteristics are obtained at temperatures above 77K.
- FIG. 10A is a top view of a superconducting layer 24 (on a substrate not shown in this view) having a single grain boundary GB therein separating the superconducting grains A and B.
- the dashed lines 26 and 28 define the boundaries of the superconducting loop that is to be formed for the SQUID device. Regions of the superconducting layer outside dashed line 26 and inside dashed line 28 were removed by laser ablation to leave a loop of superconducting material 30 (FIG. 10B).
- This superconducting loop is located on the substrate 32, and includes a first portion 30A and a second portion 30B separated by two grain boundary regions GB.
- two tunnel barrier or weak link devices are formed in the superconducting loop 30, thereby producing the SQUID.
- FIG. 11 shows the response of the SQUID of FIG. 10B to an applied magnetic field, and is based on measurements made of the device. A current was produced in the superconducting loop and a magnetic field H was applied parallel to the plane of the grain boundaries and perpendicular to the plane of the superconducting layer 24.
- FIG. 11 shows the interferometer response of the SQUID of FIG. 10B to the applied magnetic flux for three values of bias current I b , I b2 , I b3 through the SQUID loop.
- the precise periodicity and the large modulation depth as a function of magnetic field clearly demonstrates the usefulness of these grain boundary junctions in the fabrication of devices, such as the SQUID of FIG. 10B.
- the small B c ( ⁇ 1) for these grain boundary junctions eliminates the need for an external resistance shunt to achieve nonhysteretic SQUID operation.
- a planar spiral coupling coil comprised of all high-T c superconducting materials can be made on a separate chip. After individual testing of the coil, the coil and the SQUID of FIG. 10B can be positioned to achieve optimum coupling of magnetic fields from the coil to the SQUID. This provides a device with enhanced sensitivity and reproducibility.
- the critical current of the boundaries is less than that of the adjoining grains, while the temperature and magnetic field dependence of the critical currents of the grain boundaries indicate that the boundaries are comprised of regions of weak and strong coupling.
- the grain boundaries can be used to form structures such as SQUIDS and provide a natural way to develop more advanced tunneling structures that can be used for scientific or practical applications.
- the structures of this invention can be used to provide enhanced devices for many purposes, including infrared sensors and coherent arrays for detection and transmission of millimeter waves and linear junction arrays for voltage standards.
- a coherent array can be fabricated from a number of tunnel devices formed using the same grain boundary as the tunnel barrier or weak-link barrier in each device. Since each device can be made very small and have the same properties with respect to the superconducting grains and the grain boundary that is used as the tunnel barrier, coherent arrays having enhanced properties can be envisioned. Further, since the location and orientation of the grain boundaries can be precisely controlled, design layouts for a plurality of these devices are easily realized.
- the invention is specifically directed to devices and methods using controlled imperfections (such as grain boundaries) in a superconducting layer where the superconducting layer is preferably a high-T c superconductor of the type exhibiting a crystalline structure including copper oxide planes.
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Ceramic Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Superconductor Devices And Manufacturing Methods Thereof (AREA)
- Inorganic Compounds Of Heavy Metals (AREA)
- Superconductors And Manufacturing Methods Therefor (AREA)
Description
- This invention relates to devices employing high-Tc superconducting materials, and more particularly to simple, practical devices employing these materials, and to methods for making these devices. The devices are planar structures employing selected grain boundaries in the high-Tc superconducting materials as weak link or junction barriers. Such structures are reproducibly made with good operating margins.
- Recently, the remarkable discovery by J.G. Bednorz and K.A. Müller, reported in Z. Phys. B. - Condensed Matter 64, 189 (1986) and Europhysics Letters, 3, 379 (1987) completely changed the direction and importance of superconducting technology. Their discovery was that certain metallic oxides can exhibit superconducting transition temperatures considerably in excess of 23K. These materials are often termed "High-Tc Superconductors". Since the initial discoveries of Bednorz and Mueller, a vast amount of research and development has been undertaken around the world to further study these types of superconducting materials in order to extend even farther the temperature range over which the materials are superconducting, as well as to understand the basic mechanisms for superconductivity in this class of materials. sp. Bednorz and Müller first showed superconducting behavior in mixed copper-oxides, typically including rare earth and/or rare earth-like elements and alkaline earth elements, for example La, Ba, Sr, ..., and having a perovskite-like structure. Materials including the so called "1-2-3" phase in the Y-Ba-Cu-O system have been found to exhibit a superconducting transition temperature in excess of 77K. R.B. Laibowitz and co-workers were the first to achieve and describe a method for making thin films of these materials. These thin film structures and methods for making them are described in Phys. Rev. B, 35, 8821 (1987). In this technique, a vapor transport process is used in which the components of the superconducting film are vaporized and deposited on a substrate in an oxygen atmosphere, after which the deposited film is further annealed.
- Another paper describing thin films of these high-Tc superconductors, and specifically high critical currents in these materials, is P. Chaudhari et al., Phys. Rev. Lett. 58, 2864, June 1987. Chaudhari et al. described epitaxial high-Tc superconducting films formed on substrates such as SrTiO₃, in which the critical current at 77K was in excess 10⁵ A/cm².
- Other references generally describing the deposition of films or layers of high-Tc superconducting materials include EP-A-0 288 711 and EP-A-88810600.2. The first of these references describes a plasma spray coating process while the second describes a method for coating a substrate, as by spraying from solution, and then patterning the coated film to eventually produce a patterned layer of high-Tc superconducting material.
- Epitaxy of high-Tc superconducting films has been accomplished on several substrates, including SrTiO₃. In particular, superconducting films capable of carrying high critical currents have been epitaxially deposited as noted in a paper by P. Chaudhari et al., published in Physical Review Letters, 58, 2684, June 1987.
- The initial work of Bednorz and Müller has been extended to include other copper oxide compositions which exhibit high temperature superconductivity. These other compositions typically do not include a rare earth element, but instead include an element such as Bi. A representative material is one in the system Bi-Sr-Ca-Cu-O which exhibits a drop in electrical resistance at about 115K and a transition to zero resistance at 80K. Recently, C. Michel and co-workers reported superconductivity in the non-rare earth containing BiSrCuO system with transition temperatures as high as 22K, cf. C. Michel et al., Z. Phys. B-Condensed Matter, 68, 412 (1987). A new BiSrCaCuOx composition was then found by Maeda and Tanaker to exhibit high transition temperatures with a resistivity completion in the 80K range and a well defined resistivity decrease at about 115K. This work was reported by these authors in a preprint, which is to be published in the Japanese Journal of Physics.
- The copper oxide superconducting materials exhibiting transition temperatures in excess of about 30K are generally known as "high-Tc superconductors", and will be referred to in that manner throughout the specification. This designation is meant to include both the materials having rare earth or rare earth-like elements in their crystalline structure, as well as the more recently reported materials which do not contain rare earth or rare earth-like elements. Generally, all these materials are copper-oxide-based superconductors having Cu-O planes that appear to be primarily responsible for carrying the supercurrents, where the copper oxide planes are separate or in groups separated by the other elements in the compositions.
- The advent of high temperature superconductivity should lead to numerous applications of junction devices operating at temperatures much high than those that have been achieved with superconducting devices fabricated from conventional superconductors. However, fabrication of workable devices has not been easy. The first such report of an operable device, in this situation a SQUID device described by R. Koch et al., utilized a film of high-Tc superconductor in which high energy beams were used to produce two localized constrictions to form weak link connectors between high-Tc superconductors. In this manner, a superconducting loop having weak link regions was created and operated successfully as a SQUID. This first high-Tc superconducting device and the method for making it are described in EP-A-0 286 891. Although it has been experimentally established that high-Tc superconducting copper oxides, such as YBa₂Cu₃O7-x can be reproducibly prepared as thin films, a well defined, all high-Tc single junction exhibiting Josephson tunneling currents has not been successfully fabricated. In such a device, two superconducting layers comprised of high-Tc superconductors are separated by a thin (1 ... 5 nm) layer which operates as a tunnel barrier. An oxide material can be used for the barrier layer. However, the high Tc copper oxide superconductors, whether fabricated as films or bulk samples, require annealing in an oxygen atmosphere at high temperatures, typically about 900°C. This high-temperature processing makes it extremely difficult, if not impossible, to deposit a counter electrode comprised of high-Tc superconducting material over the very thin insulating tunnel barrier. Generally, the high temperature processing severely degrades the junction quality. Such processing is also incompatible with most of the conventional lithographic patterning processes.
- Another feature of these high-Tc superconducting materials is their extremely short coherence length, which is a measure of the distance over which the superconducting pairing extends. In these high-Tc superconductors the coherence lengths are typically 1 ... 3 nm, in contrast with coherence lengths of 100 nm in conventional prior art superconducting materials. Such low coherence lengths represent another technical obstacle to making either planar function or weak link type tunnel barriers in, for example, microbridge Josephson junction devices. In weak link devices, a very narrow constriction operates as a weak link barrier between two large superconducting regions to provide Josephson-like characteristics.
- However, because the coherence length is so small in high-Tc superconductors, the geometrical constriction must have a dimension of the order of the coherence length in order to exhibit weak-link characteristics. Such narrow constrictions cannot be reliably produced. When planar junctions are formed, it is also very difficult to reliably deposit tunnel barrier layers having thicknesses of the order of the coherence length (about 1 nm) of high-Tc superconductors.
- Accordingly, it is a primary object of the present invention to provide a practical device employing high-Tc superconducting materials where the aforementioned problems are avoided.
- It is another object of the present invention to provide a method for reproducibly making practical junction and weak-link superconducting devices employing only high-Tc superconducting materials.
- It is another object of this invention to provide a device and method for making the device employing high-Tc superconducting materials in a planar configuration wherein the weak-link or junction region can be precisely located with a defined orientation.
- In the practice of this invention, junction devices or weak link devices are fabricated using a grain boundary between two high-Tc superconducting grains. These grain boundaries are very narrow (about the order of the unit cell in the materials, i.e., about 1 nm, and their electrical properties (such as resistance) can be readily varied to provide different device properties. In particular, a planar structure is provided utilizing an epitaxial film of high-Tc superconducting material deposited on a substrate having defined and predetermined grain boundaries therein. In this manner, the grain boundaries in the substrate are reproducibly formed in the epitaxial superconductor film. Stated another way, epitaxy maps the polycrystalline structure of the substrate into the high-Tc superconductor film.
- It is recognized that grain boundaries have been used to provide potential barriers for the flow of electrons thereacross in prior superconducting devices. Such devices have been called boundary layer Josephson junctions and have been described in the following references:
- M. Ito et al., Japanese Journal of Appl. Physics, 21, No. 6, pp. L375-376, June 1982.
- M. Ito et al, Appl. Phys. Lett., 43, (3) p. 314, August 1983.
- T. Inamura et al, Jap. Journal of Appl. Phys., 21, Supplement 21-1, pp. 313-318, 1982.
- The devices described in these references use the grain boundaries that randomly occur when a superconductor film is deposited on a substrate. These superconductors are generally designated BPB films because they are comprised of Ba, Pb, and Bi oxide combinations having a perovskite-type structure. These references do not teach a way to controllably make grain boundary junction devices whose characteristics can be well controlled and which can be reproducibly formed with uniform properties. As noted, these references describe devices in which a random formation of randomly oriented grains occurs in materials having low transition temperatures of about 13K.
- In further contrast with these and other references, the devices of the present invention are made in an epitaxial layer of high-Tc superconducting material. Generally, epitaxy is thought of with respect to single crystal material rather than polycrystalline materials of the type used for the substrate and the superconducting film in the devices of this invention.
- M. Suzuki et al. describes the formation of planar Josephson-type devices using crystalline layers of BPB in J. App. Phys. 53, (3), p. 1622, March 1982. In this structure, two superconducting layers of BPB are separated by an insulating tunnel barrier comprised of an insulating oxide having the same crystal structure as BPB.
- Such device structures have not been possible using high Tc superconducting materials, for the reasons described above with respect to the high temperature processing and very short coherence length in these new superconductors.
- Accordingly, it is another object of the present invention to provide practical devices utilizing selected grain boundaries in high-Tc superconducting materials.
- It is another object of this invention to provide processing techniques for reproducibly making grain boundary superconductive devices employing high-Tc superconducting materials, wherein the device properties are uniform and wherein the device structures are planar and easily and reproducibly fabricated.
- It is another object of this invention to provide improved devices employing high-Tc superconductors, wherein the design of such devices and the techniques for making them effectively utilize features found in nature which may otherwise be considered obstacles.
- This invention relates to improved devices utilizing high-Tc superconducting materials and uniform, reproducibly created grain boundaries in such films for the fabrication of the devices. It is recognized that grain boundaries have been utilized in the prior art as tunnel barriers in the work relating to BPB oxides, and that the possible presence of grain boundaries leading to Josephson tunneling currents was mentioned in EP-A-0 286 891 However, the present invention seeks to provide devices and methods for producing these devices which are controllable and reproducible to define grain boundary devices in high-Tc Josephson materials having small coherence lengths. Further, the location, orientation, and number of these new grain boundary devices can be predetermined, and the properties of each of the devices can be adjusted during fabrication.
- In a preferred embodiment, a substrate is prepared having at least one grain boundary therein, which grain boundary is to be reproduced in an overlying layer of high-Tc superconductor. The layer of high high-Tc superconductor is then epitaxially deposited on the substrate in order to reproduce in the superconducting layer the grain boundary present in the substrate. This defines the location and orientation of the grain boundary in a controlled manner. After this, the superconducting film is patterned to leave at least one superconducting region on each side of the grain boundary, these superconducting regions being used as electrodes for current flow across the grain boundary. High energy beams or excimer laser ablation can be used to define the superconducting regions that are to function as the electrodes for these superconducting devices. After this, the superconducting regions are electrically contacted and appropriately biased to have current flow across the grain boundary which functions as a potential barrier. A plurality of devices of this type can be arranged in any manner to produce an array of such devices, a SQUID, etc.
- These and other objects, features, and advantages will become apparent from the following more particular description in which the preferred embodiments are described by way of example with reference to the attached drawings, in which:
- FIGS. 1-4 schematically illustrate a process wherein a grain boundary device is fabricated in an epitaxial layer of high-Tc superconducting material.
- FIG. 5 schematically illustrates electrical connections to the device produced by the technique illustrated in FIGS. 1-4.
- FIG. 6 is a schematic illustration of a layer of high-Tc superconducting material having two grains A and B separated by a grain boundary. Regions I, II, and III are defined by the dashed lines.
- FIG. 7 is an illustration of the layer of high-Tc superconducting material shown in FIG. 6, where different regions I, II and III have been processed in grains A and B to illustrate the invention.
- FIGS. 8 and 9A - 9C illustrate electrical properties of the regions I, II, and III shown in FIG. 7. More particularly, FIG. 8 is a plot of critical current Ic versus temperature for the three regions I, II, and III, while FIGS. 9A - 9C illustrate plots of critical current Ic versus applied magnetic field H for the three regions I, II, III, respectively.
- FIGS. 10A and 10B illustrate a process for fabricating a SQUID comprised of 2 grain boundary devices in a superconducting loop of high-Tc superconducting material.
- FIG. 11 is a plot of critical current Ic versus applied magnetic field H for the SQUID illustrated in FIG. 10B, for three different bias currents.
- In the practice of this invention, superconducting devices comprised of a single layer of high-Tc superconducting material can be made in a planar geometry utilizing grain boundaries for tunnel barriers or weak link connections between superconducting grains. In contrast with the prior art, this can be done reproducibly and controllably since the grain boundaries can be produced in the superconducting layers in a manner in which the orientation and location of the grain boundary are predetermined.
- The general process steps include the preparation of the substrate having at least one grain boundary defined therein with respect to the orientation and location of the grain boundary. After this, a high-Tc superconducting layer is epitaxially deposited on the substrate (or on a thin interface layer epitaxially grown on the substrate) in order to produce in the superconducting layer a grain boundary corresponding in location and orientation to the grain boundary in the underlying substrate. After this, the superconducting layer is patterned to leave regions of superconducting material on either side of the grain boundary in order to produce a device having superconducting regions (electrodes) separated by the grain boundary. Electrical contacts can then be made to the superconducting regions for connection to appropriate biasing sources. As will be seen, the properties of the grain boundary can be adjusted and multiple devices can be fabricated along a single grain boundary or along several grain boundaries.
- Referring more particularly now to FIGS. 1-4, the general technique for producing a planar grain boundary device is illustrated. In this technique, there is no subsequent processing step which would interfere with the material properties of any of the component parts of the device, and the structure that is obtained is planar.
- The grain boundary functions as a Josephson tunneling barrier or weak link connection, and is typically about 1 nm in width in these high-Tc superconducting materials. More generally, the grain boundary width is of the order of the unit cell of these high-Tc superconductors. In FIG. 1,
substrate 10 includes two single crystal grains A and B separated by a grain boundary GB. This grain boundary is approximately 1 nm in width and is schematically illustrated by the stipled region between the crystalline grains A and B. - In FIG. 2, a
layer 12 high-Tc superconducting material has been epitaxially deposited on thesubstrate 10 using, for example, known techniques. These techniques include vapor deposition as by evaporation or sputtering from multisources as described in the aforementioned articles in the names of R. B. Laibowitz, P. Chaudhari and others. Because thesuperconducting layer 12 is epitaxially formed on thesubstrate 10, it will have crystalline regions A and B coextensive with those in thesubstrate 10, and a grain boundary GB having the same orientation and location as the grain boundary in theunderlying substrate 10. - In FIG. 3, the
superconducting layer 12 is patterned, for example by using photons or high energy particles, represented by thearrows 14. This patterning can be done in a variety of ways, and is used to define regions of thesuperconducting layer 12 which are to be left in their superconducting state while the irradiated portions are either physically removed or converted to a nonsuperconducting (i .e., normal) state. In order to render an irradiated region nonsuperconducting, the technique described in EP-A-0 286 891. This technique is more fully described in two publications by G.J. Clark et al., appearing in Appl. Phys. Lett. 51, (1987) at pages 139 and 1462, and comprises the use of high energy beams to cause damage in high-Tc superconducting materials. This damage can change the properties of the material from superconducting to normal and even to a nonsuperconducting, insulating state having an amorphous structure. Examples of high energy beams that can be used for this purpose are directed ion beams comprising ions such as oxygen, As, Kr, etc. The ion beam can be directed to selected areas of the superconducting layer through the use of a mask. An example of a SQUID device made by this technique is described in R. H. Koch et al., Appl. Phys. Lett., 51, p. 200 (1987). - High energy beams providing energies in the range of about 250 keV - 2 or 3 MeV will typically provide enough damage to alter the properties of high-Tc copper oxide superconductors. However, it is within the practice of this invention that patterning can be accomplished by energy beam irradiation where the superconducting material is not totally converted to a nonsuperconducting state, but rather has its critical transition temperature Tc lowered appreciably with respect to the nonirradiated regions of the superconducting layer.
- Another technique for patterning high-Tc superconducting layers is the use of excimer ablative photodecomposition in which ultraviolet radiation impinges on the superconducting layer to ablate (i.e., blow away) the irradiated regions.
- Ablative photodecomposition will occur if the energy fluence of the UV radiation is sufficiently high that the threshold for ablative photodecomposition is exceeded. In this process, the ablation occurs with substantially no thermal effect to the surrounding nonirradiated regions. This is a particularly good technique, as the surrounding regions will have the same superconducting properties after patterning has occurred.
- FIG. 4 illustrates the structure that remains after patterning. Here, a thin strip of the
superconductor 12 is left on thesubstrate 10, thesuperconductor 12 including grains A and B separated by the grain boundary GB in the superconducting material. As is apparent, it is a planar structure wherein grains A and B can be used for electrical contacts, the current flow being substantially normal to the plane of the grain boundary. - Thus, the general steps of this process include the provision of a substrate having at least one grain boundary therein whose location and crystal orientation are predetermined, the epitaxial deposition of a layer of high-Tc superconducting film on the substrate to establish in the superconducting film a grain boundary corresponding to that in the substrate, patterning of the superconductor to leave superconducting regions separated by a portion of the grain boundary, and contacting the superconducting regions with the appropriate electrical sources. One example of the final structure is shown in FIG. 5.
- In FIG. 5,
electrical contacts 16 are made to superconducting grains A and B and a bias source, represented bybattery 18 is attached thereto for providing a current flow across the grain boundary GB in the superconducting layer. - The substrate materials are selected from those materials on which an epitaxial layer of high-Tc superconducting film can be deposited. Examples of suitable substrates include SrTiO₃ substrates for epitaxial deposition thereon of high-Tc superconducting materials such as YBa₂Cu₃O7-x. Other suitable substrates will be apparent to those of skill in the art, the substrates being generally chosen to have sufficient lattice match with the desired high-Tc superconducting material that the superconducting material can be epitaxially deposited thereon.
- Techniques exist in the art for providing substrates having controlled grain boundary growth. For example, a grain boundary with a controlled misorientation can be obtained by forming a bicrystal from two oriented single crystals. The bicrystal is grown by sintering two single crystal pieces in a powder compact. During sintering, the single crystal pieces grow at the expense of the smaller surrounding grains until the single crystals impinge on each other to form a single grain boundary. This technique has been used to form the bicrystal of SrTiO₃ at a sintering temperature of 1600°C. Alternatively, a bicrystal can be formed by hot pressing two single crystal pieces together. While both techniques can be used to form a long, well-defined grain boundary, the advantage of the second method is the ability to form a straight grain boundary that is free of pores.
- As was described, the grain boundary forms a tunneling barrier or weak link between the superconducting grains to which electrical contact is made. The critical current density and tunneling characteristics of the grain boundary can also be modified with a low temperature (less than 400°C) annealing step in a controlled gas atmosphere. Both inert and reducing gasses, such as He and Ar, as well as reactive gasses such as CO₂ are effective for this purpose. An inert gas annealing step acts to reduce the oxygen content of the superconductor film. A CO₂ anneal will promote the formation of BaCO₃ in a film of YBa₂Cu₃O7-x, where BaCO₃ is an insulator. Since the activation energies for diffusion and solid state reaction of grain boundaries are typically lower than for the rest of the lattice, an optimum set of annealing temperatures and times exist for which the transition temperature Tc and normal state resistivity of the grain boundary are altered while leaving the corresponding properties of the adjacent superconducting grains relatively unaffected.
- The principles of this invention were demonstrated by the fabrication of several Josephson junctions using a grain boundary in the high-Tc superconductor YBa₂Cu₃O7-x. For this, a polycrystalline layer of high-Tc superconductor having a large grain size in the plane of the film was fabricated. This superconducting material was epitaxially grown on a substrate of SrTiO₃. Several substrates of polycrystalline SrTiO₃, having grains as large as 200-300 µm, were used on which the YBaCuO superconducting films were deposited.
- These large grain SrTiO₃ substrates were prepared by sintering cold-pressed pellets of fine-grained powder (average grain size approximately 2.5 µm) in air at temperatures in the range 1600-1650°C, for at least 48 hours. These sintering conditions cause exaggerated grain growth to occur, which leads to the formation of very large grains in the dense pellets (ρ/ρth ≧ 99%)
- The strontium titanate powder was prepared by reacting high purity powders of strontium carbonate and titanium dioxide at 1450°C until a single phase material is obtained.
- To fabricate a single Josephson junction containing a well defined grain boundary as the weak-link bridge, high-Tc YBa₂Cu₃ films were epitaxially deposited onto the large grain polycrystalline SrTiO₃ substrates. The details of superconductor film deposition and post deposition annealing are those which have been described previously by R.B. Laibowitz, P. Chaudhari, et al. After this annealing step the superconducting films were epitaxially aligned with the grain orientation of the substrate, resulting in a large-grained superconducting film. This is illustrated in FIG. 6 where the
superconducting film 20 is epitaxially aligned with thesubstrate 22. Large grains A and B are produced in thesuperconducting film 20, where the grains are separated by the grain boundary GB. - In order to define the geometry and dimensions of a grain boundary junction device and its electrode pads, the technique of laser ablation was used. Grains A and B were patterned by excimer laser ablation as described hereinabove in order to make three lines I, II, and III, as illustrated in FIG. 7. In these examples, the substrate-superconducting film combination was mounted into a computer controlled stepping stage and irradiated with a demagnified image of variable size rectangular aperture. This technique can be used to pattern high Tc superconducting films in dimensions ranging from several centimeters in length to submicron dimensions in width without any degradation in critical temperature Tc and critical current density Jc.
- In these examples, three narrow lines I, II, III having dimensions of about 20 µm x 80 µm x 0.5 µm thickness were patterned in
superconducting film 20. This step removed the superconducting materials in the irradiated regions. The structure produced is shown in FIG. 7 where line I is totally in grain A, while line III is totally in grain B. However, line II straddles the grain boundary GB. In laboratory demonstrations, the width of the lines was varied from approximately I µm to approximately 2 µm. The length of the line within a grain was approximately 40 µm, while the length of the line crossing the grain boundary was varied between 2 and 40 µm. - The electrical characteristics of the three lines I, II, and III are shown in FIGS. 8 and 9A - 9C. In FIG. 8 the critical current Ic is plotted against temperature for current flow in each of the three lines I, II and III. From this FIG. it is apparent that line II, containing a grain boundary, always has a lower critical current than that lines I and III.
- FIGS. 9A - 9C more dramatically illustrate the essential features of a Josephson weak link junction in line II, in contrast with lines I and III. These FIGS. plot critical current Ic versus applied magnetic field H for each of the three lines I, II and III. The plots in FIGS. 9A and 9C are similar, while the plot in FIG. 9B clearly illustrates the presence of the grain boundary junction in line II. The junction resistance here is of the order of a few ohms and its capacitance is estimated to be a fraction of 1 picofarad. The Stewart McCumber parameter is of the order of 1 for these samples. Therefore, the hysteresis in the I-V curves for these samples is quite small, in sharp contrast to the conventional overlap junctions with higher capacitance made by separating two superconducting layers with an insulating layer of, for example, an oxide material.
- In the practice of the present invention, the number of grain boundaries, their orientations and their locations in the superconducting layer are known in advance of patterning for device formation. This is a major distinction over the randomness with respect to location and orientation of prior art grain boundary devices using other types of superconductors. Another major distinction is with respect to the superconducting grain size that is used in the present invention in contrast with that of prior art grain boundary devices. In the present invention, the grain sizes are typically hundreds of micrometers, in contrast with an average of about 10 microns grain size for prior art devices. Because the grains in the present invention are very large, it is easy to isolate grain boundaries for patterning and formation of devices. This cannot be achieved in the prior art where the grain boundaries are extremely short and randomly oriented.
- Another advantage of the present devices in contrast with prior art grain boundary devices relates to the uniformity of properties along the grain boundaries. In the present invention, long grain boundaries can be produced so that the same grain boundary can be used when patterning several devices, or an array of devices. Since the use of the same grain boundary as a barrier in more than one device helps to ensure the uniformity of individual device properties, the quality of circuits and arrays produced from devices made in accordance with the present invention can be significantly greater than those using prior art structures. For example, in a SQUID device, a superconducting loop is formed including at least 2 weak links or tunnel barriers. In the present invention, it is possible to construct the superconducting loop in such a way that multiple tunnel barriers are formed using the same grain boundary in order to make all the weak link devices in the superconducting loop have essentially identical properties. This cannot be achieved in prior art structures utilizing several grain boundaries.
- FIGS. 10A and 10B illustrate a SQUID device that was prepared in accordance with the present invention, while FIG. 11 is a plot of critical current Ic versus applied magnetic field H, for different values of bias current through the SQUID. This plot was made at 4,6K, although the same general characteristics are obtained at temperatures above 77K.
- FIG. 10A is a top view of a superconducting layer 24 (on a substrate not shown in this view) having a single grain boundary GB therein separating the superconducting grains A and B. The dashed
lines line 26 and inside dashedline 28 were removed by laser ablation to leave a loop of superconducting material 30 (FIG. 10B). This superconducting loop is located on thesubstrate 32, and includes afirst portion 30A and asecond portion 30B separated by two grain boundary regions GB. Thus, two tunnel barrier or weak link devices are formed in thesuperconducting loop 30, thereby producing the SQUID. - FIG. 11 shows the response of the SQUID of FIG. 10B to an applied magnetic field, and is based on measurements made of the device. A current was produced in the superconducting loop and a magnetic field H was applied parallel to the plane of the grain boundaries and perpendicular to the plane of the
superconducting layer 24. FIG. 11 shows the interferometer response of the SQUID of FIG. 10B to the applied magnetic flux for three values of bias current Ib, Ib2, Ib3 through the SQUID loop. The precise periodicity and the large modulation depth as a function of magnetic field clearly demonstrates the usefulness of these grain boundary junctions in the fabrication of devices, such as the SQUID of FIG. 10B. In addition, the small Bc (< 1) for these grain boundary junctions eliminates the need for an external resistance shunt to achieve nonhysteretic SQUID operation. - A planar spiral coupling coil comprised of all high-Tc superconducting materials can be made on a separate chip. After individual testing of the coil, the coil and the SQUID of FIG. 10B can be positioned to achieve optimum coupling of magnetic fields from the coil to the SQUID. This provides a device with enhanced sensitivity and reproducibility.
- What has been described herein are devices based on the first direct measurements of the critical current of a number of grain boundaries and of their adjoining superconducting grains. The critical current of the boundaries is less than that of the adjoining grains, while the temperature and magnetic field dependence of the critical currents of the grain boundaries indicate that the boundaries are comprised of regions of weak and strong coupling. The grain boundaries can be used to form structures such as SQUIDS and provide a natural way to develop more advanced tunneling structures that can be used for scientific or practical applications.
- The structures of this invention can be used to provide enhanced devices for many purposes, including infrared sensors and coherent arrays for detection and transmission of millimeter waves and linear junction arrays for voltage standards. For example, a coherent array can be fabricated from a number of tunnel devices formed using the same grain boundary as the tunnel barrier or weak-link barrier in each device. Since each device can be made very small and have the same properties with respect to the superconducting grains and the grain boundary that is used as the tunnel barrier, coherent arrays having enhanced properties can be envisioned. Further, since the location and orientation of the grain boundaries can be precisely controlled, design layouts for a plurality of these devices are easily realized.
- The invention is specifically directed to devices and methods using controlled imperfections (such as grain boundaries) in a superconducting layer where the superconducting layer is preferably a high-Tc superconductor of the type exhibiting a crystalline structure including copper oxide planes.
Claims (11)
a first superconducting electrode comprised of a first grain (A) of a high-Tc superconductor,
a second superconducting electrode comprised of a second grain (B) of a high Tc superconductor,
a grain boundary (GB) located between said first and second superconducting grains (A, B), and providing a potential barrier to the flow of electrons thereacross,
said first and second grains (A, B) being in a single epitaxial layer (12, 24) of high-Tc superconducting material,
said grain boundary (GB) having a plane substantially transverse to the plane of said epitaxial layer (12, 24) of high-Tc superconducting material, and
a crystalline substrate (10, 22, 32) on which said single epitaxial layer (12, 24) of high-Tc superconducting material is epitaxially deposited,
said substrate (10, 22, 32) having a grain boundary (GB) therein which is mapped into said epitaxial superconducting layer (12, 24) and corresponds in location and orientation to said grain boundary (GB) in the overlying epitaxial layer (12, 24) of high-Tc superconducting material.
providing a crystalline substrate (10, 22, 32) having a grain boundary (GB) therein,
forming a layer (12, 24) of high-Tc superconducting material with a grain boundary (GB) therein having a predetermined location and orientation in said superconducting material,
and which grain boundary (GB) corresponds in location and orientation to the grain boundary (GB) in the underlying substrate,
patterning said superconducting material to electrically isolate two regions therein located on opposite sides of said grain boundary (GB), and providing electrical connections (16) to said superconducting regions to permit the producing of current flow between said regions through said grain boundary (GB).
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US07/155,946 US5162298A (en) | 1988-02-16 | 1988-02-16 | Grain boundary junction devices using high tc superconductors |
US155946 | 1993-11-19 |
Publications (3)
Publication Number | Publication Date |
---|---|
EP0329603A2 EP0329603A2 (en) | 1989-08-23 |
EP0329603A3 EP0329603A3 (en) | 1989-11-08 |
EP0329603B1 true EP0329603B1 (en) | 1992-07-08 |
Family
ID=22557423
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP89810047A Expired EP0329603B1 (en) | 1988-02-16 | 1989-01-19 | Grain boundary junction devices using high-tc superconductors |
Country Status (4)
Country | Link |
---|---|
US (2) | US5162298A (en) |
EP (1) | EP0329603B1 (en) |
JP (1) | JPH0828536B2 (en) |
DE (1) | DE68901980T2 (en) |
Families Citing this family (29)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5077266A (en) * | 1988-09-14 | 1991-12-31 | Hitachi, Ltd. | Method of forming weak-link josephson junction, and superconducting device employing the junction |
US5157466A (en) * | 1991-03-19 | 1992-10-20 | Conductus, Inc. | Grain boundary junctions in high temperature superconductor films |
JPH05129671A (en) * | 1991-10-31 | 1993-05-25 | Sharp Corp | Superconducting element having magneto-resistance effect and manufacture thereof |
DE69224605T2 (en) * | 1991-11-28 | 1998-11-05 | Int Superconductivity Tech | Copper oxide superconductor, process for its production and copper compound used |
CA2084556C (en) * | 1991-12-06 | 1996-12-24 | So Tanaka | Method for manufacturing an artificial grain boundary type josephson junction device |
KR940008695B1 (en) * | 1991-12-26 | 1994-09-24 | 한국과학기술연구원 | Particle-boundary-type semiconducting magnetic condenser |
US5358928A (en) * | 1992-09-22 | 1994-10-25 | Sandia Corporation | High temperature superconductor step-edge Josephson junctions using Ti-Ca-Ba-Cu-O |
JP3149996B2 (en) * | 1992-11-10 | 2001-03-26 | 財団法人国際超電導産業技術研究センター | How to make Josephson bonds |
SE506807C2 (en) * | 1994-05-03 | 1998-02-16 | Ericsson Telefon Ab L M | Device providing weak links in a superconducting film and device comprising weak links |
US5714791A (en) * | 1995-12-22 | 1998-02-03 | International Business Machines Corporation | On-chip Peltier cooling devices on a micromachined membrane structure |
US5831278A (en) * | 1996-03-15 | 1998-11-03 | Conductus, Inc. | Three-terminal devices with wide Josephson junctions and asymmetric control lines |
US6331680B1 (en) | 1996-08-07 | 2001-12-18 | Visteon Global Technologies, Inc. | Multilayer electrical interconnection device and method of making same |
US20040134967A1 (en) * | 1997-05-22 | 2004-07-15 | Conductis, Inc. | Interface engineered high-Tc Josephson junctions |
US20040266627A1 (en) * | 1997-05-22 | 2004-12-30 | Moeckly Brian H. | High-temperature superconductor devices and methods of forming the same |
JP2971066B1 (en) * | 1998-12-02 | 1999-11-02 | 株式会社日立製作所 | Superconducting single flux quantum logic circuit |
US6734699B1 (en) | 1999-07-14 | 2004-05-11 | Northrop Grumman Corporation | Self-clocked complementary logic |
JP4132720B2 (en) * | 2001-05-07 | 2008-08-13 | 独立行政法人科学技術振興機構 | Manufacturing method of quantum interference magnetometer |
US20030102470A1 (en) * | 2001-08-30 | 2003-06-05 | Evgeni Il'ichev | Oxygen doping of josephson junctions |
WO2004105147A1 (en) * | 2003-01-23 | 2004-12-02 | The Trustees Of Columbia University In The City Of New York | Method for preparing atomistically straight boundary junctions in high temperature superconducting oxides |
WO2004070853A1 (en) | 2003-01-31 | 2004-08-19 | The Trustees Of Columbia University In The City Ofnew York | Method for preparing atomistically straight boundary junctions in high temperature superconducting oxide |
WO2004073024A2 (en) * | 2003-02-06 | 2004-08-26 | Brown University | Method and apparatus for making continuous films ofa single crystal material |
US7002366B2 (en) * | 2003-08-20 | 2006-02-21 | Northrop Grumman Corporation | Superconducting constant current source |
US20050062131A1 (en) * | 2003-09-24 | 2005-03-24 | Murduck James Matthew | A1/A1Ox/A1 resistor process for integrated circuits |
US8571614B1 (en) | 2009-10-12 | 2013-10-29 | Hypres, Inc. | Low-power biasing networks for superconducting integrated circuits |
US9768371B2 (en) | 2012-03-08 | 2017-09-19 | D-Wave Systems Inc. | Systems and methods for fabrication of superconducting integrated circuits |
US10222416B1 (en) | 2015-04-14 | 2019-03-05 | Hypres, Inc. | System and method for array diagnostics in superconducting integrated circuit |
CN110462857B (en) | 2017-02-01 | 2024-02-27 | D-波系统公司 | System and method for manufacturing superconducting integrated circuits |
US20200152851A1 (en) | 2018-11-13 | 2020-05-14 | D-Wave Systems Inc. | Systems and methods for fabricating superconducting integrated circuits |
US12102017B2 (en) | 2019-02-15 | 2024-09-24 | D-Wave Systems Inc. | Kinetic inductance for couplers and compact qubits |
Family Cites Families (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4290843A (en) * | 1980-02-19 | 1981-09-22 | Texas Instruments Incorporated | Epitaxial growth of magnetic memory film on implanted substrate |
JPS59210677A (en) * | 1983-05-14 | 1984-11-29 | Nippon Telegr & Teleph Corp <Ntt> | Photodetecting element using josephson junction |
JPH0763100B2 (en) * | 1986-05-21 | 1995-07-05 | 日本電信電話株式会社 | JO Josephson junction element and its manufacturing method |
JPH01120878A (en) * | 1987-11-04 | 1989-05-12 | Seiko Epson Corp | Josephson effect element |
JPH01161881A (en) * | 1987-12-18 | 1989-06-26 | Nec Corp | Josephson element and its manufacture |
GB2215548B (en) * | 1988-02-26 | 1991-10-23 | Gen Electric Co Plc | A method of fabricating superconducting electronic devices |
US5157466A (en) * | 1991-03-19 | 1992-10-20 | Conductus, Inc. | Grain boundary junctions in high temperature superconductor films |
-
1988
- 1988-02-16 US US07/155,946 patent/US5162298A/en not_active Expired - Lifetime
- 1988-11-19 JP JP63291122A patent/JPH0828536B2/en not_active Expired - Lifetime
-
1989
- 1989-01-19 EP EP89810047A patent/EP0329603B1/en not_active Expired
- 1989-01-19 DE DE8989810047T patent/DE68901980T2/en not_active Expired - Lifetime
-
1992
- 1992-09-16 US US07/945,760 patent/US5278140A/en not_active Expired - Lifetime
Also Published As
Publication number | Publication date |
---|---|
US5162298A (en) | 1992-11-10 |
JPH0828536B2 (en) | 1996-03-21 |
EP0329603A3 (en) | 1989-11-08 |
JPH01218077A (en) | 1989-08-31 |
DE68901980D1 (en) | 1992-08-13 |
US5278140A (en) | 1994-01-11 |
DE68901980T2 (en) | 1993-02-25 |
EP0329603A2 (en) | 1989-08-23 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
EP0329603B1 (en) | Grain boundary junction devices using high-tc superconductors | |
US5087605A (en) | Layered lattice-matched superconducting device and method of making | |
Barner et al. | All a‐axis oriented YBa2Cu3O7− y‐PrBa2Cu3O7− z‐YBa2Cu3O7− y Josephson devices operating at 80 K | |
US5729046A (en) | Superconducting device having pinning regions | |
EP0286891A2 (en) | Method for making superconducting quantum interference devices using high-Tc superconductors | |
US5595959A (en) | Method of forming a high-TC microbridge superconductor device | |
US6541789B1 (en) | High temperature superconductor Josephson junction element and manufacturing method for the same | |
US5627139A (en) | High-temperature superconducting josephson devices having a barrier layer of a doped, cubic crystalline, conductive oxide material | |
Beasley | High-temperature superconductive thin films | |
US5250817A (en) | Alkali barrier superconductor Josephson junction and circuit | |
US20080146449A1 (en) | Electrical device and method of manufacturing same | |
Jia et al. | High‐temperature superconductor Josephson junctions with a gradient Pr‐doped Y1− xPrxBa2Cu3O7− δ (x= 0.1, 0.3, 0.5) as barriers | |
JPH104223A (en) | Oxide superconducting josephson element | |
US6790675B2 (en) | Josephson device and fabrication process thereof | |
Smilde et al. | Y-Ba-Cu-O/au/nb ramp-type josephson junctions | |
EP0482198B1 (en) | Superconducting tunnel junction element comprising a magnetic oxide material and its use | |
US5721196A (en) | Stacked tunneling and stepped grain boundary Josephson junction | |
KR20010067425A (en) | Ramp edge josephson junction devices and methods for fabricating the same | |
US5856205A (en) | Josephson junction device of oxide superconductor having low noise level at liquid nitrogen temperature | |
EP0545815B1 (en) | Josephson junction device formed of oxide superconductor material and process for preparing the same | |
JPH04275470A (en) | Product composed of superconductor/insulator structure and manufacture of said product | |
JP2517081B2 (en) | Superconducting device and manufacturing method thereof | |
Braginski et al. | Processing and Manufacture of Josephson Junctions: High-T c | |
EP0557207A1 (en) | Josephson junction device of oxide superconductor and process for preparing the same | |
Barnard et al. | Fabrication techniques for thin film applications of high temperature superconductors |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
PUAI | Public reference made under article 153(3) epc to a published international application that has entered the european phase |
Free format text: ORIGINAL CODE: 0009012 |
|
AK | Designated contracting states |
Kind code of ref document: A2 Designated state(s): DE FR GB |
|
PUAL | Search report despatched |
Free format text: ORIGINAL CODE: 0009013 |
|
AK | Designated contracting states |
Kind code of ref document: A3 Designated state(s): DE FR GB |
|
17P | Request for examination filed |
Effective date: 19891214 |
|
17Q | First examination report despatched |
Effective date: 19911008 |
|
GRAA | (expected) grant |
Free format text: ORIGINAL CODE: 0009210 |
|
AK | Designated contracting states |
Kind code of ref document: B1 Designated state(s): DE FR GB |
|
REF | Corresponds to: |
Ref document number: 68901980 Country of ref document: DE Date of ref document: 19920813 |
|
ET | Fr: translation filed | ||
PLBE | No opposition filed within time limit |
Free format text: ORIGINAL CODE: 0009261 |
|
STAA | Information on the status of an ep patent application or granted ep patent |
Free format text: STATUS: NO OPPOSITION FILED WITHIN TIME LIMIT |
|
26N | No opposition filed | ||
REG | Reference to a national code |
Ref country code: GB Ref legal event code: IF02 |
|
PGFP | Annual fee paid to national office [announced via postgrant information from national office to epo] |
Ref country code: DE Payment date: 20071228 Year of fee payment: 20 Ref country code: GB Payment date: 20080123 Year of fee payment: 20 |
|
PGFP | Annual fee paid to national office [announced via postgrant information from national office to epo] |
Ref country code: FR Payment date: 20080110 Year of fee payment: 20 |
|
REG | Reference to a national code |
Ref country code: GB Ref legal event code: PE20 Expiry date: 20090118 |
|
PG25 | Lapsed in a contracting state [announced via postgrant information from national office to epo] |
Ref country code: GB Free format text: LAPSE BECAUSE OF EXPIRATION OF PROTECTION Effective date: 20090118 |